Second harmonic generation (SHG) is a nonlinear optic process. When light of frequency ω is irradiated onto nonlinear crystal that adopts noncentrosymmetric space group, the doubled frequency 2ω is emitted. Frequency conversion by nonlinear optic crystal is an effective way of producing coherent light at frequencies where lasers perform poorly or are unavailable. Demand on SHG materials for IR application is rapidly growing. Chalcogenide glasses are of interest because of their infrared transparency and excellent formability, consequently being promising contenders for infrared optical fiber. Meanwhile, glasses are inherently forbidden from showing nonlinear optic (“NLO”) second harmonic generation because of they are random and the presence of macroscopic center of symmetry. For example, Silica fibers play a main role of optical switches, routers, splitters, modulators, and waveguidance in current optical communications and rapidly growing high speed broadband internet. However, its usefulness is greatly limited to be passive devices because of its lack of second-order nonlinearity. It is not an SHG NLO material. Therefore, there have been tremendous efforts to induce SHG in glass using specific treatments such as thermal electric field poling, electron beam irradiation, and so on. It does need complicated processes and any induced SHG is much weaker than normal NLO crystals.
Most of the known SHG materials are oxide compounds and they have been extensively studied. Their performance is suitable for Uv-vis region but they are inefficient in the IR region because of the absorption problem in this region. In terms of SHG susceptibility, oxide nonlinear optic materials are poorer than chalcogenide species because chalcogen atoms (S, Se, Te) are more polarizable than oxygen.
Chalcogenide or chalcopyrite compounds exhibited excellent SHG response and broad transparency through infrared region. CdGeAs2, ZnGeP2, AgGaQ2 (Q=S, Se) are among top materials. CdGeAs2 ranks first in the SHG susceptibility before the introduction of APSe6 and A2P2Se6 by the inventors as set forth below.
The application of CdGeAs2 however is very limited. The compound consists of toxic elements such as Cd and As, and it is very difficult to grow single crystals. The crystals show anisotropic thermal expansion allowing cracking. Due to the small band gap, its transparency range is only 2.4 to 17 μm, consequently Nd:YAG and GaAs laser are unavailable for use in conjunction with CdGeAs2 and only CO2 laser can be used. AgGaQ2 (Q=S, Se) are the other contenders but they show anisotropic thermal expansion problem and the sulfur analogue suffers from low SHG susceptibility. Despite this AgGaSe2 is a commercially available NLO material for the infrared.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.